In known glass-clad fiber-optic arrays, both the fiber and the cladding are made of non-absorbent material to minimize transmission loss through each fiber. If the refractive index n.sub.1 of the fiber core and the refractive index n.sub.2 of the fiber cladding are sufficiently different that EQU n.sub.1.sup.2 &lt;n.sub.2.sup.2 +1
then the acceptance half-angle for rays impinging upon the input face of each fiber is at a maximum, namely, 90.degree.. This means that light at all incident angles is transmitted by total internal reflection down the fiber (except for small losses due to Fresnel reflection at the input and exit faces). In an imaging system using such an array, the depth of field is very shallow. Moreover, in utilizing diffuse illumination to suppress the effect of scratches on a scanned original, the collection of widely off-axis rays produces noticeable image blurring.
In order to reduce the aforementioned blurring and to increase depth of field of an imaging system to a useable range, it is necessary to decrease the acceptance angle. The conventional approach has been to decrease the difference between the refractive indices of the core and cladding so that the above inequality does not hold. As this difference decreases, the acceptance angle decreases. The problem then is that light rays striking the input face of the fibers at greater than the acceptance angle are not internally reflected, and instead transmit through the fiber walls to exit through another fiber. This generates flare light that severely degrades resolution in an imaging system. High quality film scanners, for example, must operate under extremely low levels of flare light to read the full dynamic range of densities and to capture the fine details of film images.
If the cladding or the interstitial spaces between fibers can be adapted to attenuate stray light within the fibers, then this problem can be alleviated to some extent. In U.S. Pat. No. 3,060,789 the cladding of a fiber is formed of a light-absorbing glass having absorption characteristics selected such that substantially only the light entering the fiber within the acceptance angle of the fiber will be transferred through the fiber, whereas light entering the fiber at angles beyond the acceptance angle will be absorbed by the cladding. Another technique is to add an array of small light-absorbing fibers located interstitially among the light-conducting fibers. The absorbing material, usually a dark glass, gradually absorbs the stray light as it wanders through the plate. The thicker the plate the more effective the absorption. Such a material is often referred to as extramural absorption, or EMA, material (see "Fiber Optics" by W.P. Siegmund, pp. 1-29 in Applied Optics and Optical Engineering, R. Kingslake, ed. Vol. IV, Academic Press: New York, 1967). The problem with such techniques is that enough unguided light is still passed through the fiber-optic array to cause significant flare. Moreover, the light absorbing cladding inherently absorbs, at each reflection, a small portion of the light which entered the fiber within its maximum acceptance angle.
The latter problem can be alleviated to some extent by providing a multiple coating or cladding on the core of the fiber (see U.S. Pat. No. 3,253,500). The fiber comprises a core of a relatively high index flint glass or the like having an inner coating or cladding of a clear relatively low index crown glass and an outer coating or cladding of a light-absorbing glass. Since the inner cladding, being a clear crown glass, will not absorb light, substantially all of the light within the acceptance angle of the fiber will be internally reflected at the interface and transferred through the fiber without in any way contacting the outer light-absorbing cladding. As with prior arrays, some unguided light, that is, light entering the fiber beyond its maximum acceptance angle, is still passed through the array to cause flare.
In the known structures cited above, the refractive indices of the core and cladding are such that the some input angles of incidence do not result in total internal reflection. U.S. Pat. No. 3,582,297 discloses a fiber-optic array structure for which the refractive indices are chosen such that all input rays are transmitted through the fiber. The problem now is that some rays enter the array through the glass cladding itself and obliquely travel through the structure, passing several times through the clad fibers and thus causing flare. According to this patent, an ion exchange process operative along the facial surface of the array darkens the cladding to a depth of 10 to 20 microns (approximately 1% of the thickness, top to bottom, of the array). This is said to absorb the radiation incident upon the cladding without causing unduly large absorption of the radiation captured by the core. However, the depth of field, as mentioned before, remains too shallow for use with high resolution image scanners.